Currently, optical input/output (I/O) is used in network systems to transmit data between computer system components. Optical I/O is able to attain higher system bandwidth with lower electromagnetic interference than conventional I/O methods. In order to implement optical I/O, radiant energy is coupled to a fiber optic waveguide from an optoelectronic integrated circuit (IC).
Typically, a fiber optic communication link includes a fiber optic transmitting device such as a laser, an optical interconnect link, and a light receiving element such as a photo detector. Currently, 10 Gbits/s optical links using an 850 nm transceiver over multi-mode fiber are implemented in network systems.
However, existing multi-mode fibers were installed for slower data rates 1 Gbit/s operation. At 10 Gbits/s modal dispersion causes optical signals to be degraded. Modal dispersion is significant in Multi-mode Fibers (MMF), widely used in shorter reach applications up to a few hundred meters. MMF modal dispersion is caused by an optical pulse that splits into several paths (modes) traveling at different speeds down a fiber. This different traveling speed and path length create Inter-symbol interference (ISI). ISI is when one bit runs into other bits, which limits the data-rate of the link or limits the link distance.
At 10 Gbits/s modal dispersion is more dramatic than at 1 Gbit/s especially over old legacy fibers. Using existing multi-mode fibers links originally installed for 1 Gbit/s to run 10 Gbits/s shows evident bandwidth limitation due to modal dispersion. In contrast good fibers could prevent error-free transmission at 10 Gbits/s over longer distance.
A current solution to overcome modal dispersion at 10 Gbits/s over a few hundred-meter legacy multi-mode links is to multiplex four individual data streams at ¼ of the data rate onto a fiber at different optical wavelengths. However, this solution is relatively expensive and is not scalable to smaller transceivers form factors (e.g., XFP).
The following are additional techniques that attempt to overcome modal dispersions at 10 Gbits/s. The techniques are to use longer wavelength, restrict the launch condition from the laser into the fiber, and use an electronic dispersion technique to compensate for signal distortions.